如何在C ++ 14中定义静态constexpr矩阵? [英] How to define a static constexpr matrix in c++14?

问题描述

我当前正在使用C ++ 14. 我想定义一个Matrix类，该类可用于定义运行时矩阵，还可以用于定义constexpr矩阵.我也想基于这样的类定义静态constexpr矩阵.

I am currently using C++14. I would like to define a Matrix class which I can use for defining runtime matrices, but also constexpr matrices. I also would like to define static constexpr matrices based on such a class.

我考虑此作为Matrix类的起点. 然后，我想写一些东西:

I consider this as a starting point for the Matrix class. Then I would like to write something as:

static constexpr Matrix<double,2,2> staticmat{0.1,0.2,0.3,0.4};

使得staticmat是constexpr并且是唯一的，是静态的.

so that staticmat is constexpr and unique, being static.

但是，为了进行初始化，我需要一个constexpr数组或一个constexpr初始化程序列表(在我发布的链接中未实现，但不会有太大变化).所以我可以这样写:

However, in order to initialise this, I would need a constexpr array or a constexpr initialiser list (not implemented in the link I posted, but not much would change). So I could write something like:

static constexpr std::array<double,4> staticmattmp{0.1,0.2,0.3,0.4};
static constexpr Matrix<double,2,2> staticmat(staticmattmp);

这将是丑陋的，因为我必须为一个定义两个东西，但是，如果可行，我可以接受.不幸的是，编译器显示unknown type name 'staticmattmp'.

This would be ugly because I have to define two things just for one, but, if it worked, I could accept it. Unfortunately the compiler says unknown type name 'staticmattmp'.

我怎么可能以一种优雅的方式解决这个问题?

How can I solve this, maybe in an elegant way?

推荐答案

我怎么可能以一种优雅的方式解决这个问题?

How can I solve this, maybe in an elegant way?

我不知道它是否优雅，但是...做了一点工作...

I don't know if it's elegant but... with a little work...

首先，定义以下using

template <typename T, std::size_t>
using getType = T;

下一次重新声明(仅声明；未定义)，如下所示Matrix

Next re-declare (declare only; not define) Matrix as follows

template <typename, std::size_t NR, std::size_t NC,
          typename = std::make_index_sequence<NR*NC>>
class Matrix;

现在将您的Matrix声明为类部分专业化，添加一个构造函数，该构造函数接收类型为T的NR*NC元素，并使用它们初始化内部的std::array

Now declare your Matrix as a class partial specialization adding a constructor that receive NR*NC elements of type T and use them to initialize the internal std::array

template <typename T, std::size_t NR, std::size_t NC, std::size_t ... Is>
class Matrix<T, NR, NC, std::index_sequence<Is...>>
 {
   public:
       using value_type = T;

      constexpr Matrix (getType<value_type, Is> ... vals)
         : values_{{vals...}}
       {}

      // other member and methods
 };

但是不要忘了将析构函数(可能也是构造函数和operator=())声明为default.

But don't forget to declare as default the destructor (maybe also constructor and operator=()).

以下是完整的C ++ 14示例

The following is a full compiling C++14 example

#include <array>
#include <type_traits>

template <typename T, std::size_t>
using getType = T;

template <typename, std::size_t NR, std::size_t NC,
          typename = std::make_index_sequence<NR*NC>>
class Matrix;

template <typename T, std::size_t NR, std::size_t NC, std::size_t ... Is>
class Matrix<T, NR, NC, std::index_sequence<Is...>>
 {
   public:
      using value_type = T;

      constexpr Matrix (getType<value_type, Is> ... vals)
         : values_{{vals...}}
       {}

      constexpr Matrix (std::array<T, NR*NC> const & a)
         : values_{a}
       {}

      constexpr Matrix (std::array<T, NR*NC> && a)
         : values_{std::move(a)}
       {}

      constexpr Matrix () = default;

      ~Matrix() = default;

      constexpr Matrix (Matrix const &) = default;
      constexpr Matrix (Matrix &&) = default;

      constexpr Matrix & operator= (Matrix const &) = default;
      constexpr Matrix & operator= (Matrix &&) = default;


      constexpr T const & operator() (std::size_t r, std::size_t c) const
       { return values_[r*NC+c]; }

      T & operator() (std::size_t r, std::size_t c)
       { return values_[r*NC+c]; }

      constexpr std::size_t rows () const
       { return NR; }

      constexpr std::size_t columns () const
       { return NC; }

   private:
      std::array<T, NR*NC> values_{};
 };

int main()
 {
   static constexpr Matrix<double,2,2> staticmat{0.1,0.2,0.3,0.4};
 }

这篇关于如何在C ++ 14中定义静态constexpr矩阵?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！